Currently, liquid crystal display apparatuses that have been widely available utilize a liquid crystal panel in TN (Twisted Nematic) mode. In recent years, for the purpose of overcoming drawbacks in TN mode of a narrow viewing angle and low response capabilities, liquid crystal display apparatuses using a liquid crystal panel in OCB mode have been developed and reported (Japanese Patent Laid-Open Publication No. 7-84254,—Japanese Patent Laid-Open Publication No. 9-96790, etc.)
In the liquid crystal panel in OCB mode, as disclosed in the above Japanese Patent Laid-Open Publication No. 9-96790, special processing is required for causing a transition from a spray configuration to a bend configuration (such a transition from the spray configuration to the bend configuration is hereinafter referred to as “transition”) prior to a start of video display. This processing, however, is not directly related to the present invention, and therefore is not further described herein.
Now, in the liquid crystal panel in OCB mode, as illustrated in FIG. 14, even if the above processing causes the state of OCB cells to make a transition to the bend configuration, the state returns back to the spray configuration if a voltage applied to the OCB cells continues to be lower than a constant voltage Vc (such a transition from the bend configuration back to the spray configuration is hereinafter referred to as “back transition”). Therefore, as represented by a characteristic a illustrated in FIG. 14, most liquid crystal display apparatuses using a liquid crystal panel in OCB mode limit the amplitude of a video signal so that a voltage (larger than Vc) within a range enabling the bend configuration to be kept is always applied to the OCB cells (note that this is a case of normally white). However, if the amplitude of the video signal is limited as such, the maximum transmittance of the liquid crystal panel is small (Ta in FIG. 14). Consequently, the maximum luminance (luminance in white display (bright display)) of the liquid panel is decreased, there by causing inconveniences, such as that a desired luminance cannot be obtained.
However, with a high voltage being periodically applied to the OCB cells, back transition does not occur even if the voltage applied to the OCB cells temporarily becomes lower than Vc, which is disclosed in Japanese Patent Laid-Open Publication No. 11-109921 and Japanese Liquid Crystal Society Journal, Apr. 25, 1999 (Vol. 3, No. 2) P.99 (17) through P.106 (24). With this being used for displaying an image of one frame, one frame period is divided into a period for displaying the image and a period for applying a high voltage. Therefore, as represented by a characteristic b illustrated in FIG. 14, the range of the voltage applicable to the OCB cells as a video signal was able to be expanded to Vw, which is lower tan Vc, Such a driving scheme is hereinafter referred to as “anti-back-transition driving”. Also, the high voltage regularly applied to the OCB cells for the purpose of preventing back transition is hereinafter referred to as “anti-back-transition voltage”. According to the anti-back-transition driving, the maximum transmittance of the liquid crystal panel can be increased (Tb illustrated in FIG. 14). As a result, the maximum luminance of the liquid crystal display apparatus can be increased. Note that the inventors have confirmed that preventive effects against back transition are increased as the anti-back-transition voltage is increased and also as a ratio of a time period with respect to one frame period (a ratio of a time period during which the voltage is maintained with respect to one frame period) is increased. Note that the preventive effects against back transition represent herein how hard back transition occurs when parameters (the ratio of the time period of applying the anti-back-transition voltage with respect to one frame period and a liquid crystal temperature), which cause fluctuations in a back transition occurrence condition, are fluctuated.
However, when the above-described anti-back-transition driving is performed, it was found that the magnitude of the anti-back-transition voltage required for preventing anti-back-transition and the ratio of the time period with respect to one frame period are fluctuated by various factors, such as the temperature of the liquid crystal panel (more precisely, liquid crystal).
Therefore, the inventors have investigated, by way of example, a relation between the ratio of the time period of applying the anti-back-transition voltage with respect to one frame period and the temperature of the liquid crystal panel. Consequently, in a case of an OCB liquid crystal material used by the inventors, as denoted by a long and short dashed line in FIG. 15, it is found that as the liquid crystal temperature is increased, the ratio of the time period of applying the anti-back-transition voltage with respect to one frame period is increased. Therefore, for example, if the ratio of the time period of applying the anti-back-transition voltage with respect to one frame period is at the minimum as required at room temperature, back transition occurs when the liquid crystal temperature is high, making it impossible to perform video display. Therefore, in order to make it possible to perform video display even when the temperature of the liquid crystal panel is increased, one measure can be thought to apply the anti-back-transition voltage at a sufficiently large ratio so as not to cause back transition even when the temperature of the liquid crystal panel is 80° C., for example. However, as the ratio of the time period of applying the anti-back-transition voltage (voltage corresponding to black display) with respect to one frame period is increased, the maximum luminance is disadvantageously decreased. Note that the luminance referred to herein is a brightness felt by people, and is none other than a time integral of the transmittance within one frame period. That is why the maximum luminance is decreased as the ratio of the applying time (black display) with respect to one frame period is increased.
Furthermore, the inventors also confirmed that the preventive effects against back transition can be increased by increasing the magnitude of the anti-back-transition voltage. Therefore, one measure can be thought such as that, in order to make it possible to perform video display even when the temperature of the liquid crystal panel is increased, a sufficiently large voltage is applied as the anti-back-transition voltage so as not to cause back transition even when the temperature of the liquid crystal panel is 80° C., for example. However, if a voltage larger than an applied voltage corresponding to black display (dark display) (Vb in FIG. 14) is applied to an OCB liquid crystal, the transmittance of the OCB liquid crystal is increased as illustrated in FIG. 14. Therefore, the maximum luminance (luminance at black display) is disadvantageously increased (that is, contrast is impaired).
Still further, the inventors also confirmed that the preventive effects against back transition can be also enhanced by increasing the applied voltage (Vw in FIG. 14) corresponding to white display. Therefore, one measure can be thought such as that, in order to make it possible to perform video display even when the temperature of the liquid crystal panel is increased, a sufficiently large voltage is applied so as not to cause back transition even when the temperature of the liquid crystal panel is 80° C., for example. However, as the applied voltage corresponding to white display is increased, the transmittance of the liquid crystal at the time of white display is decreased, thereby causing a decrease in maximum luminance.
Therefore, an object of the present invention is to provide a liquid crystal display apparatus capable of always optimally displaying video irrespectively of fluctuations in a back transition occurrence condition.